The present invention relates to a method for transporting audio/video data transmitted from a camera or a microphone, that is, media data with high quality, a transmitter and a receiver therefor.
Various connection modes are used over an IP (Internet Protocol) network, such as an intranet or the Internet. Depending on the connection mode, the usable bands of the network in terms of bit rate vary in a range of from several kilobits per second (kbps) to several hundreds of megabits per second (Mbps). In addition, the influence of other flows (transmission data) causes the usable band to vary in time. As such, transport rate control is necessary to perform media data transport across a network of the type described above. The transport rate control adjusts the transport rate in accordance with the usable transport bands.
Known protocols employed to transport media data across the Internet in real time include RTP (Realtime Transport Protocol)/RTCP (RTP Control Protocol) (H. Schulzrinne et al., “RTP: A Transport Protocol for Real-Time Applications”, RFC 1889, Internet Engineering Taskforce, Jan. 1996). According to the RTP/RTCP, a reception terminal transmits feedback information regarding, for example, the packet loss rate, propagation delay time, and jitter, to a transmission terminal, and the transport rate is determined on the basis of the feedback information. The transmission interval (notification interval) for the feedback information from the reception terminal is a fixed value, for example, five seconds.
Known methods of transport rate control using feedback information as described above include a method called TFRC (TCP Friendly Rate Control) (M. Handley et. al., “TCP Friendly Rate Control (TFRC): Protocol Specification”, RFC 3448, Internet Engineering Taskforce, Jan. 2003). According to the TFRC, the transport rate is controlled to increase until a packet loss event occurs, and the transport rate is controlled to decrease when the packet loss event has occurred. Thereby, TFRC enables update to be smoothly achieved for producing a transport rate suitable for intended media data transmission. The transport rate update interval is 1×RTT (Round Trip Time), ranging from several tens of milliseconds (ms) to several hundreds of milliseconds (ms), for example.
Another known method is a method described in Request for Comments (RFC) 2733 (J. Rosenberg et al., “An RTP Payload Format for Generic Forward Error Correction”, Internet Engineering Taskforce, Dec. 1999), which is a technique of imparting an error correction capability to RTP. More specifically, the RFC 2733 technique adds FEC data (FEC: forward error correction), which is redundant data for forward error correction, to data which is to be transmitted.
In the event of transmitting encoded data based on media data, when its transport rate is updated following the TFRC at intervals ranging from several tens of milliseconds (ms) to several hundreds of milliseconds (ms), and the encoding rate is updated every time the transport rate is updated, the encoding rate frequently varies. As such, for example, quality of images to be transmitted frequently varies, resulting in deteriorating view image quality.
In addition, when the notification interval for feedback information from a reception terminal to a transmission terminal is set short, an advantage is exhibited in that a usable transmission band can quickly be accessed, and the transmission band can be efficiently used. On the other hand, the update frequency for the transport rate in the transmission terminal is rendered high, and quality of media data is frequently varied to an extent that the media transport cannot be implemented maintaining stable quality. In addition, the degree of fairness in throughput with respect other flow is reduced. Conversely, with the notification interval for the feedback information is set long, the update frequency for the transport rate is reduced. In this case, while the transport rate is stabilized, variations in the transmission band cannot be followed, thereby disabling efficient use of the transmission band. Further, in the case where the notification interval is set long, when the usable transmission band is narrow, packets are lost in a burst-wise manner, and media data transmission quality is extremely deteriorated.
Moreover, conventionally, since the data packet size is uniform, when the transport rate is reduced, the number of the packets is reduced. Generally, in a reception terminal, a packet loss rate L is calculated by the following equation:L=1−(number of data packages arrived at a reception terminal within a time T)/(number of data packages transmitted from a transmission terminal whithin the time T)
Ordinarily, the time T is a time interval from the instant of transmission of previous feedback information to the instant of transmission of current feedback information. The equation describes that as the number of packets to be transported from the transmission terminal decreases, the packet loss of one packet causes the packet loss rate to be greatly vary. For example, assume a state where the transport rate significantly decreases, and no more than one packet can be transported within the time T. In this state, when the packet is not lost, the packet loss rate is 0%; whereas, when the packet is lost, the packet loss rate is 100%. In such the state, for the reason that an accurate network congestion state cannot secured for the reason that the packet loss rate cannot finely be observed.
Further, in a method represented by TFRC and placing primary importance on the fairness with respect to the TCP, The RTT is included in a denominator of a calculation expression that determines the transport rate. The RTT is a time required for the round trip of a packet between a transmission terminal and a reception terminal; as such it can take a value 1 ms or less when the distance between the transmission terminal and the reception terminal is short. In this state, the RTT measurement result is influenced by various delay fluctuations irrelevant to the network congestion state. For example, the delay fluctuations include a delay fluctuation in processing of an operating system that performs processing for packet transmission from the transmission terminal, and a delay fluctuation in transfer processing of a router that performs packet transfer processing.